Photochemical process for fossil fuel combustion products recovery and utilization

ABSTRACT

Sulfur dioxide (SO 2 ) and oxides of nitrogen (NO x ) are effectively and economically removed from a gaseous combustion products stream by photochemical conversion of the gaseous SO 2  and NO x  components into particulates (aerosols and mists). The reactive hydrocarbon (RHC) and oxygen deficient fossil fuel combustion products can be converted into a highly photochemically reactive RHC,SO 2 ,NO x  (NO,NO 2 ),O 2 ,H 2  O gaseous mixture by the introduction of sufficient quantities of a reactive hydrocarbon, such as an olefin, and oxygen or oxygen containing air. The reactant mixture is then irradiated with electromagnetic waves of the 1,500A to 7,500A band of the spectrum, which irradiation is followed by ammonia injection, if desired, to promote particulate formation. The particulate nitrogenous, sulfrous (&#34;nitrates&#34; and &#34;sulfates&#34;) and other particulate compounds are removed from the stream by a conventional particulate control system and the products and byproducts can be separated and converted into valuable products of economic value, such as organic and inorganic acids, fertilizers and the like.

BACKGROUND AND SUMMARY OF THE INVENTION

The usefullness of the disclosed process and apparatus lies in itsfunction of reducing SO₂ and NO_(x) emissions from fossil fuelcombustion products to below or near levels judged environmentally safeand necessary for public health protection. By the application of thedisclosed process, large quantities of high sulfur content Eastern U.S.coal reserves, now unavailable at reasonable costs for electric powergeneration because of sulfur emission air pollution regulations, couldbe made available. Since the U.S. coal reserves are judged adequate, atthe present energy consumption rate, for 300 to 500 years, theutilization of the high sulfur U.S. coal reserves in an environmentallysafe manner could substantially contribute to U.S. energy independenceby freeing substantial quantities of oil and natural gas for higher enduses, such as home heating, industrial applications and transportation.In addition, the disclosed process offers a novel approach for thesubstantial reduction of NO_(x) emissions which hitherto have beenjudged one of the most significant yet one of the most difficult airpollutants to control.

The application of photochemical process for fossil fuel SO₂ and NO_(x)combustion products air pollution control represents a significantlyuseful and novel means for increased public health protection andincreased national energy independence.

The products and byproducts of atmospheric photochemical reactions offossil fuel combustion products, SO₂ and NO_(x), and reactivehydrocarbons (RHCs), namely sulfates and nitrogen dioxide, have beenassociated with large scale chronic disease mortalities in U.S. urbanareas. These chronic diseases are the neoplasms of the respiratorysystem and gastro-intestinal tract; cardiovascular diseases such asarteriosclerotic heart disease and hypertensive heart disease; andnephritis. Some investigators have estimated that as much as 15 to 20%of all mortalities in U.S. metropolitan areas, or 200,000 to 300,000deaths annually, may be associated with fossil fuel combustion productsair pollution. Reduction of fossil fuel air pollution is therefore ofparamount importance.

The principal primary reactants of atmospheric photochemical reactionsare reactive hydrocarbons, NO_(x), principally NO, and SO₂. It is wellknown that when a mixture of these primary reactants is irradiated withsunlight or electromagnetic waves in the ultraviolet (UV) range, a hostof photochemical reactions take place which result in the creation ofsecondary gaseous reactants such as NO₂, ozone, peroxides, aldehydes,SO₃ and several others which may themselves undergo additionalphotochemical and other chemical reactions to result in photochemicallyrelated particulate and gaseous products. The most important, andperhaps the best known ones, are sulfates and nitrates, includingnitric-nitrous and sulfuric acids; polycyclic organic matter, includingbenzo(a) pyrene, a suspected carcinogen; and particulate nitrogenouscompounds perhaps the best known of which are peroxyacetyl nitrate(PAN), nitrosamines and related compounds.

The characteristic and most important feature of the products ofphotooxidation, from an emission control point of view, is that most ofthem are particulate in form and thus can be readily removed byconventional particulate control methods. The photochemical processconverts, and thus consumes the primary reactants, SO₂, NO_(x) and RHCs.

The definition of hydrocarbon reactivity in photochemistry is important.For purposes of this disclosure, the concept of reactivity includes therate of primary reactants disappearance (HC, SO₂ and NO) or the rate ofcreation or appearance of the products of photochemical reactions suchas NO₂ or oxidants. Classes of hydrocarbons, and individual hydrocarbonswithin the same class, have different reactivities. On a relative scaleof reactivity, paraffins, straight chain saturated hydrocarbons, havethe lowest reactivity. Aromatics and oxygenates have higher reactivitiesthan paraffins. Unsaturated hydrocarbons, such as olefins, among themdienes, have the rank in the highest ranges of the hydrocarbonreactivity spectrum. The general rule is that the higher carbon numbermembers of a class are more reactive than the lower carbon numbermembers. Reactivity of hydrocarbons dramatically increases as the numberof double bonds increase.

Several mechanisms have been offered to explain the details of theRHC-SO₂ -NO_(x) -O₂ H₂ O system photochemical reactions, although somereactions are yet imperfectly understood. There is however, generalagreement that the principal photochemical reactions involve chainreactions with a number of free radicals acting as intermediaries. Someof the important radicals have been identified as alkyl (R.), and acyl(RCO), peroxyalkyl (ROO., including HO₂.), peroxyacyl ##STR1## andacylate ##STR2##

For example, for purposes of illustration, an oversimplified andincomplete reaction mechanism can be postulated as follows:

(1) NO₂ + (UV) → NO + O

(2) o + o₂ + rhc → ro₂., etc.

(3) RO₂. + NO₂ → PAN, etc.

(4) RO₂. + SO₂ → RO₂ SO₂ + RO., etc.

(5) O + O₂ → O₃

(6) no + o₃ → no₂ + o₂

(7) so₃ + h₂ o → h₂ so₄

(8) o₃ + no₂ → no₃ + o₂

(9) no₃ + no₂ → n₂ o₅

(10) n₂ o₅ + h₂ o → 2 hno₃

it can readily be noted that if ammonia is present in a moistenvironment, ammonium salt of sulfuric and nitric acids readily form asshown:

(11) H₂ SO₄ + 2NH₄ OH → (NH₄)₂ SO₄ + 2H₂ O

(12) hno₃ + nh₄ oh → nh₄ no₃ + h₂ o

it is believed, however, that in presence of moisture hydroxyl radical(HO.) reactions dominate, in which case NO and SO₂ conversions mayproceed as shown:

(13) HO. + SO₂ → HOSO₂

(14) hoso₂ + o₂ → hoso₂ o₂

(15) hoso₂ o₂ + no → no₂ + hoso₂ o

(16) hoso₂ o + rh → h₂ so₄ + r,

where RH represents an organic compound or radical.

There are, in addition, other postulated sulfur dioxide conversionmechanisms, namely, the chemical reactions of photochemically excitedstates of sulfur dioxide. It is well known that the major sunlightabsorption of SO₂ occurs within a relatively strong band which extendsfrom 3400A to 2400A. Absorption within this band results initially inthe generation of an excited single state in SO₂. This absorption may berepresented as shown:

(17) SO₂ + hν (3400- 2900A) → ¹ SO₂

a second "forbidden" absorption region of SO₂ extends from 4000 to3400A. The absorption of sunlight within this region results in thedirect excitation of SO₂ to an excited triplet species, ³ SO₂, asdepicted below:

(18) SO₂ + hν (4000-3400A) → ³ SO₂

through a series of steps, the singlet excited ¹ SO₂ can be transformedinto the triplet state, ³ SO₂. The excited triplet state can bechemically quenched as shown:

(19) ³ SO₂ + M → products,

where M is some molecule other than SO₂.

Some compelling evidence suggests that the major, if not the exclusive,chemically reactive entity in the photochemistry of pure SO₂ is the ³SO₂ molecule. Of great interest to the photochemical reactions of theexcited triplet state SO₂ are the ³ SO₂ -quenching rates of atmosphericcompounds, such as are shown in Table 1 below:

Table 1. Summary of Quenching Rate Constant Data for Sulfur DioxideTriplet Molecules with Various Atmospheric Components and CommonAtmospheric Contaminants at 25° C. (H. W. Sidebottoms et al)

    ______________________________________                                                          Quenching rate - Kg liter/                                  Compound          mole-sec × 10.sup.-8                                  ______________________________________                                        Nitrogen          0.85 ± 0.10                                              Oxygen            0.96 ± 0.11                                              Water             8.9 ± 1.2                                                Carbon monoxide   0.84 ± 0.04                                              Carbon dioxide    1.14 ± 0.07                                              Nitric oxide      741 ± 33                                                 Ozone             11.0 ± 1.2                                               Methane           11.6 ± 0.16                                              Propylene         850 ± 87                                                 CIS-2-butene      1340 ± 98                                                ______________________________________                                    

Comparison of the rate constants in Table 1 indicates that both nitricoxide and reactive hydrocarbons (propylene and CIS-2-butene) have ordersof magnitude greater quenching rates than the other compounds.

Fossil fuel burning power plant combustion products contain SO₂, NO,NO₂, N₂, H₂ O, CO₂, CO and very little oxygen and RHCs. The compositionof flue gasses from burning one percent sulfur-bearing fuel oil may be:

So₂ = 600 ppm; NO = 200 ppm; NO₂ = 15 ppm; N₂ = 7.5 × 10⁵ ppm; H₂ O =1.3 × 10⁵ ppm; CO₂ = 1.2 × 10⁵ ppm;

O₂ ≅ 0.0 ppm; and RHC ≅ 0.0 ppm. If excess air is used in combustion,the O₂ concentration may be higher.

It can be readily seen that to make the above flue gassesphotochemically reactive, analogous to a RHC-NO_(x) - SO₂ - O₂ -H₂ Osystem, a RHC and oxygen must be added in sufficient quantities prior toirradiation. An indispensible element of the disclosed invention is thecreation of the proper photochemically reactive RHC-NO_(x) -SO₂ - O₂ -H₂O gaseous system most favorable to SO₂ conversion. This system may takethe form, for example, of a simple gaseous mixture of the individualcomponents. Another essential feature of the disclosed invention is theconversion of gaseous SO₂ and NO_(x) into particulates such as acidmists and other particulates by electromagnetic irradiation mostfavorable to free radical formation and SO₂ excitation, preferably hν(4000-3400A) and hν (3400-2400A) with peaks at or near 3700A and 2850A,respectively, followed by conventional particulate removal. In apreferred embodiment of the invention, an optional introduction ofgaseous ammonia into the irradiated stream leaving the reactor iseffected prior to the particulate removal step. In another preferredembodiment, the recovered nitrogeneous and sulfurous compounds aresubjected to further treatment and separation for the recovery ofvaluable products and byproducts which may at the same timesignificantly reduce the already comparably low solid and liquid wastes.In fact, compared to the solid waste disposal problems of the lime andlimestone wet scrubbing flue gas desulfurization processes, the wasteproblems incidental to the desulfurization and denoxification (NO_(x)removal) of the disclosed photochemical process are relatively minor.

The disclosed process can be used either primarily for SO₂ or NO_(x)emissions control, or both. Furthermore, it offers the additionaladvantage of controlling at the source the major reactants ofatmospheric photochemical reactions, thus the reduction of all productsand byproducts of atmospheric photochemical reactions.

In addition to the photochemical air pollution control of fossil fuelcombustion products such as SO₂, NO_(x) and others, the presentinvention provides a process that can be used for the photochemicalproduction of organic and inorganic acids from fossil fuel combustionproducts, the photochemical production of fertilizers from fossil fuelcombustion products, and the photochemical removal and recovery of othervaluable byproducts from fossil fuel combustion products.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates a simple form of the system ofapparatus embodying the disclosed invention.

The combustion sources which comprise sulfur-bearing fossil fuelcombustion products are shown diagrammatically at 1 and are passed intoa conventional particulate control device or system 2, which may consistof a gravity settling chamber, a cyclonic separator, electrostaticprecipitator, or other similar control devices, and the system 2 may inpractice consist of any desired combination of these well known controldevices.

Particulate materials such as soot, ash, metal oxides and the like areremoved from the control system 2 at line 22. In certain instances, itmay be desired to optionally by-pass some or all of the fuel combustionproducts around the control system 2, as shown by bypass line 24, withthe combustion products in such event being conveyed directly into heatexchanger 3.

The heat exchanger 3 functions to cool the combustion gases to thedesired temperature, preferably between 100° and 300° C. After passagethrough the heat exchanger, reactive hydrocarbons or reactivehydrocarbon mixtures are added at line 26, and oxygen or air isintroduced at line 28. A fan or compressor 4 is provided to move thecombustion products with the reactive hydrocarbons down stream in thesystem, with the pressure of the combustion products stream beingincreased to between 1 and 10 atmospheres. Depending on systemconditions, a certain portion of the combustion products stream canbypass the fan or compressor 4 as shown at 30 for reentry into the linedownstream of the compressor.

The combustion products stream under pressure is passed to aphotochemical irradiation chamber or reactor shown diagrammatically at 5for irradiating the stream as above described. Downstream of the reactor5, ammonia is introduced through line 32 to promote aerosol formation,although this step can be bypassed if desired as indicated by line 34.

A further heat exchanger 6 is provided for cooling the stream to promoteaerosol formation, that is, condensation, agglomeration, coalescence,and the like, with the stream flowing to a further particulate controldevice or system 7 which may consist of individual or combinations ofwet scrubbers, fabric filters, electrostatic precipitators, or othercollection devices known in the art. The particulate materials,including nitrogenous, sulfurous and other compounds are removed at line36, and such compounds will be aqueous if wet scrubbers are employed inthe control system 7. As noted by bypass line 38, depending uponoperating conditions, the heat exchanger 6 may be bypassed and thecombustion products stream passed directly to the particulate controldevice 7.

After passing from the control device 7, the cleaned flue gases, whichare desulphurized and denoxified, are directed to stack 9 for passage tothe atmosphere. Depending upon the quality of the cleaned flue gases,all or a portion of the flue gases may be recirculated by fan orcompressor 8 back into the system in advance of the photochemicalreactor 5 for further treatment.

It will be understood that all of the equipment schematicallyillustrated in the drawing, FIG. 1, and above referred to iscommercially available, and no invention resides therein. Rather, theinvention resides in the particular use and arrangement of suchequipment in accordance with the foregoing description.

Some of the major considerations in the election of the operatingconditions of the reactor (temperature, pressure, reactant mixturecomposition and reactant ratios) and the selection of particulatecontrol system for the removal of the particulate products ofphotochemical reactions are as follows.

With respect to the selection of particulate control systems, wetscrubbing is preferred because some products of photochemical reactions,e.g. liquid peroxyacetylnitrates, are known to be explosion hazards andbecause of the reasonably high fine particulate collection efficiency ofsuch system. The wet system has the additional advantage of removingportions of gaseous pollutants such as NO₂.

In regard to pressure, because most of the products and byproducts ofphotochemical reactions are wanted in particulate form, which are formedfrom gaseous pollutants, according to the theorem of Chatelier, theformation of particulates is promoted by increasing the reactorpressure. On the other hand, increasing pressure increases thedissociation energy of NO₂ and RHCs and the general process energyrequirement (compression), not to mention capital investment.

With respect to temperature, the reactor temperature has to besufficiently low to prevent outright combustion of the reactivehydrocarbons and the decomposition particulate products but high enoughto promote free radical formation at the minimum electromagnetic energyconsumption. However, after most of the photochemical reactions havetaken place, particulate formation is enhanced by lowering thecombustion products stream temperature.

With respect to reactant ratios/RHC requirements, it is well known thatRHCs, when undergoing photochemical dissociation, produce, in a chainreaction fashion, unpredictable number and kind of free radicalsdepending on radiation intensity, the species of RHC and otherconditions. For each combustion products stream, where composition isalso highly variable, there is a range of RHC/SO₂ and RHC/NO_(x) ratiowhich favors photochemical conversion. Since the theoretical predictionof the optimum reactant ratios is impossible and because no generalrules can be derived from isolated and non-standardized experiments, thebest rule to follow is to experimentally determine the optimum reactantratios for each particular application.

In regard to reactor residence time/Electromagnetic reactor energyrequirements, since the electromagnetic energy absorption follows Beer'sabsorption law, it can be stated that, for a fixed intensity radiation,the energy absorption efficiency can be increased by increasing thereactor cell length and by increasing the concentration of the substanceof interest. The photochemical conversion efficiency, related toabsorbance, can be increased by increasing the residence time of thegaseous streams in the reactor which can also be achieved byrecirculation of part of the cleaned stream, which minimizes energy losson particulates formed in the reaction. The electromagnetic energyrequirement for the photochemical reactions are expected to beconsiderably lower than for similar systems without the benefit from thepresence of free radicals derived from reactive hydrocarbons.

In regard to further considerations of the system parameters, radiationintensity dramatically increases the rate and efficiency ofphotochemical conversion reactions. A problem, not entirelyinsurmountable, is anticipated by particulate deposition on the outersurface of UV irradiators. Such deposition significantly reduces lightenergy transmission thereby resulting in undesirable loss of energy.

The invention as herein-above set forth is embodied in particular formand manner but may be variously embodied within the scope of the claimhereinafter made.

It is understood that various modifications may be introduced into theembodiments illustrated and described without exceeding the scope of theinvention.

I claim:
 1. A process for treatment of a gaseous mixture containingNO_(x) and SO₂ wherein X is 1 or 2, which comprises:a. addition ofreactive olefinic hydrocarbon and oxygen to said mixture in sufficientquantity to form an enriched mixture favorable to free radical formationand photochemical conversion of said NO_(x) and SO₂, b. irradiation ofsaid enriched mixture with electromagnetic radiation having a wavelength of from about 1500A to about 7500A to form free radicals andproduce particulate formation, and c. separation of particulate materialfrom said irradiated mixture.
 2. The process as defined by claim 1,wherein said gaseous mixture is the product of combustion fossil fuels.3. The process as defined by claim 1, wherein said gaseous mixturecontains in addition to NO_(x) and SO₂ gaseous O₂ and gaseous H₂ O. 4.The process as defined by claim 1, wherein oxygen is added to saidmixture prior to said irradiation step (c).
 5. The process as defined byclaim 1, wherein said reactive olefinic hydrocarbon is a straight chainhydrocarbon.
 6. The process as defined by claim 5, wherein said reactivehydrocarbon is selected from the group consisting of propylene andCIS-2-butene.
 7. The process as defined by claim 1, wherein saidelectromagnetic radiation is in a range of 4000-3400A and 3400-2400A. 8.The process as defined by claim 1, wherein after irradiation step (c)gaseous ammonia is added to said system to promote particulateformation.
 9. The process as defined by claim 1, wherein said additionand irradiation are performed on a continuously flowing gaseous streamof fossil fuel combustion products.